Númenor is the island kingdom destroyed in J.R.R. Tolkien's Middle-earth history.
Númenor was given to the Men who had helped the Elves during the wars against Morgoth. Its people became powerful mariners, and the island stood in the Great Sea between Middle-earth and the Undying Lands. Over time, fear of death and the desire for immortality contributed to rebellion against the Valar.
Under King Ar-Pharazôn, Númenórean forces sailed west in an attempt to challenge the power of the Valar. The island was then destroyed in a cataclysm, and its survivors founded kingdoms in Middle-earth, including Gondor and Arnor.
Númenor is not the same as Valinor, the realm of the Valar, nor Beleriand, a region destroyed much earlier in the First Age. Its story is central to The Silmarillion and to Tolkien's wider history.